Replace java.util.Random with kotlin Random object (#4364)

This also improves randomness by avoiding to reinitialize the random
number generator repeatedly from a seed based on the current time.
Typically, if the number generator is reinitialized repeatedly at
non-random times (like multiple times in a row), then generated numbers
have a higher chance of repeating.

The Kotlin Random object is only initialized once, using the best seed
available for the current Android version.

app/src/main/java/com/keylesspalace/tusky/util/StringUtils.kt

@@ -3,15 +3,14 @@
 package com.keylesspalace.tusky.util
 
 import android.text.Spanned
-import java.util.Random
+import kotlin.random.Random
 
 private const val POSSIBLE_CHARS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
 
 fun randomAlphanumericString(count: Int): String {
     val chars = CharArray(count)
-    val random = Random()
     for (i in 0 until count) {
-        chars[i] = POSSIBLE_CHARS[random.nextInt(POSSIBLE_CHARS.length)]
+        chars[i] = POSSIBLE_CHARS[Random.nextInt(POSSIBLE_CHARS.length)]
     }
     return String(chars)
 }